1. Field of the Invention
This invention relates generally to apparatus for tempering glass, and is particularly concerned with apparatus of the type, sometimes referred to as "blastheads," in which air or other cooling fluid under pressure is directed onto opposite surfaces of a sheet of glass subsequent to heating of the glass for quenching or rapidly cooling the sheet of glass.
2. Description of the Prior Art
The tempering of glass sheets by first heating the sheet and then suddenly cooling the heated sheet increases the mechanical strength of the glass sheet, which, in and of itself, increases the safety and the use of the glass. In addition, however, the safety is increased because the tempered glass, when broken, disintegrates and shatters into small, relatively dull and harmless particles instead of into large, sharp slivers as is the case with untempered glass.
Conventional glass tempering processes include systems wherein untempered glass sheets are first conveyed into a furnace and heated to a desired temperature, after which the heated glass sheets are conveyed to a quenching apparatus wherein the temperature of the glass is rapidly reduced to complete the tempering process. Conventionally, the quenching apparatus is of the type sometimes referred to as a "blasthead" wherein air, or other fluid, under pressure, is directed from nozzles onto opposite surfaces of the glass sheet to cool the glass.
Typically, blastheads include a pair of plenum chambers with opposed banks of nozzles projecting from the respective plenum chambers for directing air under pressure onto opposite surfaces of a sheet of glass disposed between the opposed banks of nozzles. It is, of course, important with this type of apparatus to maintain uniform flow and pressures through the nozzles from the respective plenum chambers -- it being highly undesirable to subject the heated glass sheet to any significant variation in pressure over its surface.
In some systems, the glass sheets are disposed in a horizontal plane as they are conveyed through the quenching apparatus, in which case, one plenum chamber is disposed above the other, with the lower plenum chamber having upwardly directed nozzles, and the upper plenum chamber having downwardly directed nozzles so that the upwardly and downwardly directed nozzles respectively direct fluid flow onto the lower and upper surfaces of the glass sheets. See, for example, U.S. Pat. No. 2,188,401.
In other systems, the untempered glass sheets are suspended in a vertical plane by tongs on a carrier or other conveying element. The vertically disposed sheets are conveyed by the carrier from the furnace to the blasthead. The blasthead plenum chambers are oriented vertically, that is, with their opposed banks of nozzles located on opposite sides of the vertically disposed sheet of glass. See, for example, U.S. Pat. Nos. 2,134,797; 2,146,390; 3,353,946 and 3,402,038.
Particularly in systems in which the glass sheets are disposed vertically for both heat treatment and quenching in a tempering process, the glass sheets are generally conveyed step-by-step through the heat treating furnace and quenching apparatus so that the sheets are, for a time, stationary while disposed between the opposed banks of nozzles of the blasthead. Since the glass sheets are at a high temperature as they move into position between the blasthead nozzles, delivery of high pressure fluid at fixed points on the glass can produce irregularities in the surface of the glass. This condition can be alleviated somewhat by causing the blasthead nozzles to move relative to the stationary glass sheets during the quenching operation. See, for example, U.S. Pat. Nos. 2,263,679 and 2,724,215 wherein the blasthead is caused to oscillate or reciprocate relative to the glass sheet during the quenching operation.